1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus, for example, a copying apparatus or a laser beam printer, and an electrostatic recording apparatus. More particularly, the present invention relates to an image forming apparatus for obtaining an image by developing an electrostatic latent image formed onto an image bearing member by toner.
2. Related Background Art
FIG. 5 is a schematic block diagram showing an image forming apparatus, and the operation thereof will be simply described.
First, an original G is put on an original mount 10 so that the surface to be copied of the original G faces downward. Next, a copy button is pressed, thereby starting the copying operation. The original G is scanned in the direction of an arrow by a unit 9 constructed integrally by a lamp for irradiating the original, a short focus lens array, and a CCD sensor while being irradiated by the unit 9. The short focus lens array forms an image of original surface reflection light of the irradiation scanning light, and the formed image is inputted to the CCD sensor.
The CCD sensor is made up of: a light receiving unit; a transfer unit; and an output unit.
An optical signal is converted into a charged signal by the CCD light receiving unit, and the charged signal is synchronized with a clock pulse and sequentially transferred to the output unit by the transfer unit. In the output unit, the charged signal is converted into a voltage signal and amplified, and the impedance of the amplified signal is decreased, thereby outputting the signal. The voltage signal (analog signal) obtained in the above-mentioned manner is subjected to well-known image-processing, converted into a digital signal, and transmitted to a printer unit.
In the printer unit, the foregoing image signal is received and an electrostatic latent image is formed as follows. A photosensitive drum 1 is driven to rotate at a predetermined circumferential velocity in the direction of an arrow A while setting a spindle la as a center. In the rotating process, the surface is first subjected to a charging process uniformly so as to have a voltage of about -650 V by a charger 3. The uniformly charged surface is scanned by light of a solid laser element for emitting light switched on/off in reply to the image signal with a rotary polygon mirror 104 (in FIG. 6) that rotates at a high speed, and thereby sequentially forms an electrostatic latent image for which the surface potential is attenuated to about -200 V in correspondence with the original image onto the surface of the photosensitive drum 1.
FIG. 6 is a schematic block diagram showing the structure of a laser scanning unit 100 for scanning laser beams in the image forming apparatus. When scanning the laser beams with the laser scanning unit 100, a solid laser element 102 is first flickered at a predetermined timing on the basis of an inputted image signal by a light emitting signal generating device 101.
The laser beams irradiated from the solid laser element 102 are converted into luminous flux that are substantially in parallel to each other with a collimator lens system 103. Further, the beams are scanned toward the direction of an arrow C with the rotary polygon mirror 104 rotating in the direction of an arrow B. The image is also formed onto a spot of a surface to be scanned 106 of a photosensitive drum or the like through an f.theta.-lens group of 105a, 105b, and 105c.
The laser beams are scanned as mentioned above and, therefore, exposure distribution corresponding to one scanning of the image is formed on the surface to be scanned 106. Further, if the surface to be scanned 106 is scrolled in the direction vertical to the scanning direction by a predetermined quantity in every scanning, exposure distribution can be obtained on the surface to be scanned 106 in response to the image signal.
Next, a developing step will now be described. Generally, there are four kinds of developing methods as follows.
(1) A method of coating non-magnetic toner onto a sleeve by a blade or the like, and coating magnetic toner onto the sleeve by magnetic force and carrying them, thereby developing the toner to the photosensitive drum 1 in a non-contact state (one-component non-contact developing)
(2) A method of developing the toner coated as mentioned above to the photosensitive drum in a contact state (one-component contact developing)
(3) A method of mixing magnetic carriers with toner particles, using them as the developer, and carrying them by magnetic force, thereby developing an image to a photosensitive drum in a contact state (two-component contact developing)
(4) A method of developing the foregoing two-component developer in a non-contact state (two-component non-contact developing)
It is easy to obtain a halftone image having high resolution. Consequently, there has been frequently used, for an image forming apparatus requiring a high picture quality, the two-component contact developing method such that the mixture of the toner particles and the magnetic carriers are used as the developer, and the image is developed in the contact state with the photosensitive drum such as a full-color copying apparatus.
A developing device 4 has a developer container 16 as shown in FIG. 7. The inside of the developer container 16 is partitioned to a developing chamber (first chamber) R1 and an agitating chamber (second chamber) R2 by a partitioning wall 17. Supply toner (non-magnetic toner) 18 is contained in a toner storage chamber R3. A supply port 20 is provided to the partitioning wall 17, and the supply toner 18 in an amount corresponding to consumed toner is dropped and supplied into the agitating chamber R2 through the supply port 20.
On the other hand, a developer 19 is contained in the developing chamber R1 and agitating chamber R2. The developer 19 is a two-component developer having non-magnetic toner and magnetic particles (carriers) (as a mixing ratio, the ratio of the non-magnetic toner is set to about 4% to 10% by weight). The non-magnetic toner has a volume mean particle diameter of about 5 to 15 .mu.m. The magnetic particles are comprised of ferrite particles that have been resin-coated, resin particles to which a magnetic substance has been dispersed, or the like. As for the magnetic particles, the weight mean particle diameter is equal to 25 to 60 .mu.m, the volume resistivity is equal to 10.sup.6 to 10.sup.13 .OMEGA..multidot.cm, and the transmittivity of the magnetic particles is equal to 2.5 to 5.0.
An opening portion is provided to a region that is close to the photosensitive drum 1 of the developer container 16, and a developing sleeve 11 is provided so as to be projected toward the outer side from the opening portion. The developing sleeve 11 is rotatably assembled in the developer container 16. The outer diameter dimension of the developing sleeve 11 is equal to 32 mm. The circumferential velocity of the developing sleeve 11 is equal to 280 mm/sec and the developing sleeve 11 is rotated in the direction of an arrow in the drawing. An interval Da between the developing sleeve 11 and the photosensitive drum 1 is arranged so as to have a distance of almost 500 .mu.m. The developing sleeve 11 is made of non-magnetic material, and a magnet 12 serving as magnetic field generating means is fixed into the developing sleeve 11.
The magnet 12 has: a developing magnetic pole S1; a magnetic pole N3 located downstream from the developing magnetic pole S1; and magnetic poles N2, S2, and N1 for carrying the developer 19. The magnet 12 is arranged in the developing sleeve 11 so that the developing magnetic pole S1 almost faces to the photosensitive drum 1. The developing magnetic pole S1 forms a magnetic field near a developed portion between the developing sleeve 11 and photosensitive drum 1, and a magnetic brush is formed by the formed magnetic field.
A blade 15 is disposed in the upper direction of the developing sleeve 11 at a predetermined interval T between the blade 15 and the developing sleeve 11. The interval T therebetween is equal to almost 800 .mu.m, and the blade 15 is fixed to the developer container 16. The blade 15 is made of non-magnetic material such as aluminum and SUS316, and regulates the layer thickness of the developer 19 on the developing sleeve 11.
A carrying screw 13 is contained in the developing chamber R1. The carrying screw 13 is rotated in the direction of an arrow. By the rotation, the developer 19 in the developing chamber R1 is carried toward the longitudinal direction of the developing sleeve 11.
A carrying screw 14 is contained in the storage chamber R2. The carrying screw 14 is rotated in the direction of an arrow, and the toner is carried along the longitudinal direction of the developing sleeve 11.
The developing sleeve 11 bears the developer at the position near the magnetic pole N2. The developer 19 is carried toward the developed portion in accordance with the rotation of the developing sleeve 11. When the developer 19 reaches the portion near the developed portion, the magnetic particles of the developer 19 are coupled by the magnetic force of the magnetic pole S1 and then raised from the developing sleeve 11, thereby forming the magnetic brush of the developer 19.
As a developing system, a reversal developing system is adopted. A DC voltage and an alternating voltage are applied to the developing sleeve 11 from a power source (not shown). In the example shown in the drawing, a voltage of -500 V as a DC voltage and a voltage of Vpp=2,000 V as an alternating voltage are applied, and a rectangular wave of Vf=2,000 Hz is applied.
Generally, developing efficiency is increased and an image has a high quality when applying an alternating voltage. On the contrary, there might be caused a danger that fog is easily generated. Therefore, a potential difference is normally set between the DC voltage applied to the developing device 4 and the surface potential of the photosensitive drum 1, thereby realizing the prevention of fog. In the drawing, the potential for preventing fog is a potential difference of 150 V between a potential of -650 V charged uniformly first and a DC voltage proportion of -500 V applied to the developing sleeve 11.
On the other hand, a contrast potential is a potential to adhere the toner to the photosensitive drum 1 from the developing sleeve. The contrast potential is a difference potential of 300 V between a potential of -200 V that has been exposed and attenuated and a DC potential proportion of -500 V of the voltage that is applied to the developing sleeve 11. As discussed above, the toner image formed on the photosensitive drum 1 is electrostatically transferred onto a transfer material by a transfer charger 7. After that, the transfer material is electrostatically separated by a separate charger 8 and conveyed to a fixing device 6, thereby thermally fixing and outputting the image.
The surface of the photosensitive drum 1 after transferring the toner image is subjected to a process for removing adhered stains such as remaining transfer toner by a cleaner 5, and repeatedly used for image formation.
In recent years, concern for the environment has been raised and, as for a charging method without using corona discharge, a direct charging member has been used. Particularly, an injection electrifying system is extremely excellent as a system by which a discharge amount is extremely small at the time of charging the surface of a photosensitive drum. According to the injection electrifying system, electric charges are injected to a trap potential which the surface material of the photosensitive drum has, by a contact charging member, thereby charging, or an electric charge injecting layer in which conductive particles are dispersed is provided to the surface of the photosensitive drum and charges are applied to the conductive particles by the contact charging member, thereby charging.
In this case, the volume resistivity of the surface layer of the photosensitive drum is set to about 10.sup.9 to 10.sup.14 .OMEGA..multidot.cm. It is known that when superimposing an alternating electric field to a bias to be applied to the contact charging member, charging efficiency is increased and the long life of the contact charging member can be attained. It is also known that it is preferable to use the following alternating electric field. That is, as for the alternating electric filed which is superimposed, a peak-to-peak voltage, that is, Vpp is equal to 500 V or more, preferably, equal to 700 V or more, and a frequency thereof lies within a range of 300 to 5,000 Hz, preferably, 500 to 2,000 Hz.
However, there is a problem in that fog is generated in the image and only the outputted image having low density is obtained according to the injection electrifying system in the following case. That is, according to the injection electrifying system, in order to realize the long life of the contact charging member and obtain preferable charging performance, the alternating electric field is superimposed to the bias applied to the contact charging member by using the photosensitive drum whose surface layer as described above is adjusted so as to have a volume resistivity of about 10.sup.9 to 10.sup.14 .OMEGA..multidot.cm and the photosensitive drum is uniformly charged. After that, an electrostatic latent image is formed by exposing the image and developed by the foregoing two-component developing method, thereby forming the image.
The inventors of the present invention have made various investigations about phenomena in which the fog is generated and a decrease in image density occurs. As a result, it is determined that the phenomena occur by injecting charges to the photosensitive drum whose surface layer has the volume resistivity of about 10.sup.9 to 10.sup.14 .OMEGA..multidot.cm from the magnetic carriers upon developing.
The following is necessary to accomplish a preferable injection electrifying for the photosensitive drum whose surface layer has the volume resistivity of about 10.sup.9 to 10.sup.14 .OMEGA..multidot.cm as schematically explained above. That is, as for magnetic particles for injection electrifying, there are used ferrite particles whose volume resistivity is equal to 10.sup.10 .OMEGA..multidot.cm or less and whose weight mean diameter is equal to almost 100 .mu.m or less, preferably, equal to 15 to 50 .mu.m or the like. The magnetic particles in a coated state of almost 100 mg/cm.sup.2 or more are born on the surface of the charging sleeve including therein the magnet. While holding an interval of almost 500 .mu.m between the charging sleeve and the photosensitive drum 1 and rubbing the magnetic particles to the charging sleeve, there is applied the DC voltage which is almost equal to a target potential to be charged. There is also applied the alternating voltage as the voltage Vpp which is equal to 500 V or more, preferably 700 V or more, and the frequency which lies within a range of 300 to 5,000 Hz, preferably, 500 to 2,000 Hz.
In this instance, a state where the magnetic particle layer comes into contact with the photosensitive drum is very important in order to certainly bear and carry the magnetic particles without transferring the magnetic particles from the charging sleeve to the photosensitive drum (adhering carriers), to uniformly charge the surface of the charging sleeve, and to assure a sufficient injecting time. Factors to determine the state are the magnet included in the charging sleeve as described above, in particular, a position of the magnetic pole (referred to as a charging pole, hereinafter) arranged close to a portion opposite to the photosensitive drum and a half-value width as shown in FIG. 4A (angle of an area showing a half value of the maximum magnetic flux density of the magnetic pole).
In other words, in order to enable the sufficient charging time and the uniform charging without adhering carrier, the position of the magnetic pole of the charging pole is made extremely close to the portion that is the closest to the photosensitive drum (for example, 5.degree. from the closest portion on the upstream side in the rotational direction of the photosensitive drum) and the half-value width is widened.
As expressed above, according to the two-component developing method, the developer comprising the non-magnetic toner and the magnetic carriers having the volume resistivity of about 10.sup.6 to 10.sup.13 .OMEGA..multidot.cm is born and carried on the rotatable developing sleeve including therein the magnet. While rubbing the developer to the photosensitive drum at the portion that is almost opposite to the photosensitive drum under the developing bias as described in the above-mentioned example, only the non-magnetic toner is transferred to the photosensitive drum without transferring the magnetic carriers (adhering the carrier) and the image is developed so as to enable a sufficient image density and assure image quality. In this instance, as with the foregoing charge, important parameters for developing performance include the position of the magnetic pole (referred to as a developing pole, hereinafter) which is close to the opposite portion of the photosensitive drum, and the half-value width as shown in FIG. 4B (angle of the area that is indicative of a 1/2-value of the maximum magnetic flux density of the magnetic pole).
That is, to carry the developer certainly without adhering any carrier, a peak position (having a maximum magnetic carrying force) of the developing pole is made extremely close to the position opposite to the photosensitive drum (for example, 2.degree. from the closest portion on the upstream side in the rotational direction of the photosensitive drum), and the half-value width is widened so that the developing time (abutting width for developing) for image density is increased.
If it is exemplified that the volume resistivity of the magnetic carriers is equal to that of the magnetic particles for charging or less, in the structure of the developing device, a rubbing state of the portion opposite to the photosensitive drum is very similar to that of the injection charger, thereby causing an electric charge injection phenomenon in the developed portion.
Upon causing the electric charge injection phenomenon in the developed portion, in both the white portion (white ground portion) and the black portion (black ground portion), the potentials are converged to a DC component of a voltage applied to the developing sleeve. The white portion is a portion that is uniformly charged to the photosensitive drum, and thereafter, not exposed. The black portion is a portion that is uniformly charged to the photosensitive drum and, thereafter, exposed. Therefore, it is founded that a potential difference between the white portion and the developing sleeve is decreased, the fog is generated, a potential difference between the black portion and the developing sleeve is also decreased, and consequently, the image density is dropped.
The phenomenon remarkably occurs in the foregoing case where an angle formed between the developing pole and the closest contact point of the developing sleeve and the photosensitive drum on the developing sleeve is smaller than an angle formed between the charging pole and the closest contact point of the charging sleeve and photosensitive drum on the charging sleeve, and the half-value width of the developing pole is larger than that of the charging pole.